Plants mostly
monoecious or dioecious, occasionally with perfect flowers mixed with the
pistillate ones, shrubs or more commonly small or large trees. Leaves opposite,
petiolate, lacking stipules (these fused to the petiole bases and usually not
apparent, except sometimes in A. saccharum), the leaf blades usually
palmately lobed, less commonly pinnately compound. Inflorescences terminal or
lateral toward the branch tips, sometimes axillary, ranging from small clusters
to racemes or small panicles. Flowers actinomorphic, hypogynous, the staminate
ones often perigynous. Calyces of 4 or 5(6) sepals, these sometimes fused,
often colored. Corollas absent or of 4 or 5(6) free petals. Stamens
3–8, usually strongly exserted, the filaments sometimes attached to the
margin of a nectar disk. Pistil 1 per flower, superior, of 2 fused carpels,
usually with 2 locules, flattened at right angles to the septum. Styles 2 per
flower or sometimes 1 and deeply 2-lobed, the stigmas 2. Ovules usually 2 per
locule. Fruits consisting of 2 samaras that are initially fused at the base but
break apart at maturity and are dispersed independently, each with a single
basal seed and a terminal wing. About 115 species, widespread in temperate
portions of the Northern Hemisphere and in mountains in the tropics.

Emerging
morphological and molecular evidence suggests that the Aceraceae (and
Hippocastanaceae) might best be treated within an expanded circumscription of
the Sapindaceae (Judd et al., 2002). The traditional classification is followed
here in anticipation of further studies to resolve the phylogenetic
relationships within this family complex.

Maples are
important components of many deciduous forest communities. The wood of various
species is of commercial importance for lumber, for boards and slats, for
veneers, as pulpwood for paper, and in the construction of furniture and
musical instruments. Species valued for timber production usually are divided
into two groups: “hard maples” with harder wood that is better suited for
structural uses, which include the A. saccharum complex; and “soft
maples” with more brittle wood, which include A. negundo, A. rubrum, and
A. saccharinum. Although A. saccharum is the species best known
for the use of its sap for a sugar and maple syrup, most other species also
yield sap with similar properties (but inferior in quality) to that of sugar
maple.

Numerous species
also are cultivated as ornamentals because of their interesting leaf shapes and
colors, especially their bright coloration during the autumn. A large number of
cultivars exist for some of taxa. In addition to the species treated below,
several additional maples are cultivated commonly in Missouri but have not been documented as
escapes. These include A. palmatum Thunb. (Japanese maple), a shrub or
small tree with leaves having strongly tapered tips on the lobes and relatively
small, widely spreading samaras; and A. pseudoplatanus L. (sycamore
maple), which has relatively showy flowers in small panicles that are not
produced until the trees are mostly leafed out. Settergren and McDermott (1962)
suggested that A. platanoides L. (Norway maple), a shade tree
distinguished by its milky sap and widely spreading samaras, probably had
become naturalized somewhere in Missouri, but although this species is commonly
cultivated as a shade tree in the state it has not yet been found established
outside cultivation.

Leaf blades lighter green on the undersurface than on the upper surface, but not silvery white, the central lobe broadest at the base or with nearly parallel sides; flowers produced during expansion of the leaves ... 5. A. SACCHARUM

Leaf blades relatively shallowly lobed, the lateral lobes cut 1/41/2 of the way to the base, the central lobe broadest at or just above the base; flowers with small petals; fruits with the wings 1.53.0(4.0) cm long, narrowly spreading, sometimes appearing parallel or nearly so

Flowers produced before the leaves, sessile or nearly so in dense clusters from buds along the branches (the stalks elongating greatly after flowering as the fruits mature), appearing orangish red to purplish red ... 3. A. RUBRUM